1. Field of the Invention
The present invention is directed to a process and a system to treat coke particles generated in process and heating applications while minimizing NOx emissions and minimizing required SCR catalyst volume.
2. Description of the Related Art
In many process and heating applications, cracking is utilized whereby heavy organic molecules, such as hydrocarbons, are broken down into lighter molecules, such as light hydrocarbons. An example of such a process or heating application would be an ethylene cracking furnace. The cracking process may be initiated by heat, by catalysts, or by solvents. An early thermal cracking process may be observed in Burton (U.S. Pat. No. 1,049,667) issued in 1913 titled “Manufacture of Gasolene”.
The cracking process often results in a slow deposition of coke, a form of carbon, on the reactor or vessel walls and/or on a series of serpentine tubes within the vessel or reactor. Over time, this degrades the efficiency of the process. Accordingly, a de-coking procedure is periodically utilized. The furnace or vessel is initially isolated or taken off-line from the normal process application. The accumulated coke may then be removed in a number of ways. Coke may be mechanically removed, such as by scraping or chipping. Alternatively, a fluid of hot water and/or steam is passed into and through the vessel or tubes. The steam and water are utilized to unloosen and remove the coke particles. The coke particles are thereafter removed in this manner and the vessel or furnace is then put back in to use.
The steam and/or water and coke or coke slurry must thereafter be dealt with. Some forms of coke may have value and may be reused, such as for fuel. So-called green coke may be used as fuel in refineries, cement kilns and steel industries. Other forms of coke may be used in battery terminals or other uses. Much of the coke, however, has little value; for example, it is sometimes used in filler for roadway construction and maintenance.
A later prior art development is shown in a simplified diagram in FIG. 1. A furnace, reactor or vessel 10 includes a series of internal serpentine tubes 12. A burner 14 supplied with fuel, such as natural gas, from a fuel line 16 provides heat. A portion of the heat generated in the furnace, reactor or vessel may be captured in a heat recovery unit 20. The mixture of slurry of steam and/or water and coke is removed from the furnace via a port 22. The coke particles removed from the vessel or furnace are slowly reintroduced back in to the burner 14 of the furnace via a line 24. The water and steam would be vaporized and a portion of the coke particles would be oxidized and consumed. A negative outcome of this procedure is that it generates increased NOx as emissions from the furnace 10 at an exhaust 28 depicted by arrow 26. Combustion of fossil fuels is known to generate some level of NOx emissions, which includes nitric oxide (NO) and nitrogen dioxide (NO2).
NOx may be controlled in a number of ways. Martin et al. (U.S. Pat. No. 6,003,305) illustrates an example of a method of reducing NOx products of incomplete combustion in an internal combustion engine. A selective catalytic reduction system (SCR) is disposed downstream of a flameless thermal oxidizer. SCR systems catalytically reduce NOx emissions to nitrogen and water using a catalyst, in conjunction with ammonia (NH3).
Harold et al. (U.S. Pat. No. 7,682,586) discloses an example of treatment of nitrogen oxides (NOx) in combustion flue gas with selective catalytic reduction (SCR) using ammonia and urea as reducing agents.
FIG. 2 illustrates a simplified diagram of a later prior art development following FIG. 1. A vessel, reactor or furnace 30 includes a series of internal serpentine tubes 32. A burner 34 supplied with fuel, such as natural gas, from a fuel line 36 provides heat. A portion of heat generated in the vessel or furnace may be captured in a heat recovery unit 38. The mixture or slurry of steam and/or water and coke is removed from the furnace via a port 44. The coke particles removed from the vessel or furnace are slowly introduced back in to the burner 34 via a line 46.
A selective catalytic reducer (SCR) 40 is added near the exhaust 42 of the furnace shown by arrow 48. A chosen catalyst, such as those including ammonia (NH3), would be utilized on a physical support or block having a pattern, such a honeycomb pattern. The SCR system 40 would serve to reduce the NOx emissions. In high dust situations, such as coal dust or coke particulate dust, larger SCR blocks with larger openings are required. A higher catalyst volume would be required per pound of NOx that would be treated. In some cases, two to three times the volume of catalyst would be required. This increases the size, the complexity, and the cost of the overall furnace assembly.
Assignee's prior patent, Wirt et al. (U.S. Pat. No. 8,017,084) discloses an example of a selective catalytic reduction system for heat recovery systems and fired heaters.
There remains a need to develop a process and a system to treat the coke by-products removed from a furnace, reactor or vessel efficiently.
There remains a need to develop a process and system to treat coke by-products directed to the goal of minimizing the volume of SCR required while maximizing the efficiency.